Indentation

Use spaces for indenting, 2, 3 or 4 spaces preferably. Use the same amount
of spaces for indenting as is used in the rest of the file.

Upper/lower casing

CMake commands are case-insensitiv (only the commands, not the arguments or variable names). So all the following versions work:
add_executable(foo foo.c)
ADD_EXECUTABLE(bar bar.c)
Add_Executable(hello hello.c)
aDd_ExEcUtAbLe(blub blub.c)

But this would be ugly.

In KDE the all-lowercase style is preferred. The all-uppercase style is also ok. Mixing upper- and lowercase should not be done in KDE CMake files.
Although all-lowercase is preferred, if a file is apparently in all-uppercase style, then stay consistent and also use all-uppercase in this file.

Writing CMake Find-modules

Follow the style guide from CMake when writing some FindFoo.cmake module:
readme.txt